1. Field of the Invention
The present invention provides a direct feed fuel cell for producing electrical energy by electrochemical oxidation/reduction of an organic fuel, and in particular to a direct feed methanol fuel cell system with integrated gas separation.
2. The Prior Art
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suitable for use as a fuel depending upon the materials chosen for the components of the cell and the intended application for which the fuel cell will provide electric power.
Fuel cell systems that utilize carbonaceous fuels may be divided into “reformer-based” systems (i.e., those in which the fuel is processed in some fashion to extract hydrogen from the fuel before it is introduced into the fuel cell system) or “direct oxidation” systems in which the fuel is fed directly into the cell without the need for separate internal or external processing. Most stationary fuel cells are reformer-based fuel cell systems. However, because fuel processing is expensive and requires significant volume, reformer-based systems are presently limited to comparatively high power applications. Because of their ability to provide sustained electrical energy, fuel cells have increasingly been considered as a power source for smaller devices including consumer electronics such as portable computers and mobile phones. Accordingly, designs for both reformer based and direct oxidation fuel cells have been investigated for use in portable electronic devices. Reformer based systems are not generally considered a viable power source for small devices due in part to the size, expense, and technical complexity of present fuel reformers.
Thus, significant research has focused on designing direct oxidation fuel cell systems for small applications, and in particular, direct systems using carbonaceous fuels including but not limited to methanol, ethanol and aqueous solutions thereof. One example of a direct oxidation fuel cell system is a direct methanol fuel cell system. There are several reasons why a direct methanol fuel cell (DMFC) power system is advantageous for providing power for smaller applications. First, methanol has a high energy content, thus providing a compact means of storing energy. In addition, methanol can be stored and handled with relative ease, and because the reactions necessary to generate electricity in an DMFC system occur under ambient conditions.
DMFC power systems are also particularly advantageous since they are environmentally friendly. The chemical reaction in a DMFC power system yields carbon dioxide and water as by products (in addition to the electricity produced). Moreover, a constant supply of methanol and oxygen (preferably from ambient air) can continuously generate electrical energy to maintain a continuous, specific power output. Thus, mobile phones, portable computers, and other portable electronic devices can be powered for extended periods of time while substantially reducing or eliminating at least some of the environmental hazards and costs associated with recycling and disposal of alkaline, Ni-MH and Li-Ion batteries.
The electrochemical reaction in a DMFC power system is a conversion of methanol and water to CO2 and water. More specifically, in a DMFC, methanol, which may be in an aqueous solution, is introduced to the anode face of a protonically-conductive, electronically non-conductive membrane in the presence of a catalyst. When the fuel contacts the catalyst, hydrogen atoms from the fuel are separated from the other components of the fuel molecule. Upon closing of a circuit connecting a flow field plate of the anode chamber to a flow field plate of the cathode chamber through an external electrical load, the protons and electrons from the hydrogen atoms are separated, resulting in the protons passing through the membrane electrolyte and the electrons traveling through an external load. The protons and electrons then combine in the cathode chamber with oxygen producing water. Within the anode chamber, the carbon component of the fuel is converted by combination with water into CO2, generating additional protons and electrons.
The principal electrochemical processes in a DMFC are:
Anode Reaction: CH3OH+H2O=CO2+6H++6e
Cathode Reaction: 3/2O2+6H++6e−=2H2O
Net Reaction: CH3OH+3/2O2−CO2+H2O
The methanol in a DMFC is preferably used in an aqueous solution to reduce the effect of “methanol crossover”. Methanol crossover is a phenomenon whereby methanol molecules pass from the anode side of the membrane electrolyte, through the membrane electrolyte, to the cathode side without generating electricity. Heat is also generated when the “crossed over” methanol is oxidized in the cathode chamber. Methanol crossover occurs because present membrane electrolytes are permeable (to some degree) to methanol and water.
The voltage output of a single fuel cell may not be sufficient to provide appropriate power to the desired application. Given the strict form factor limitations and increasingly demanding power requirements of portable electronic equipment, most applications require much higher voltages than what a single, typical DMFC can provide—which is on the order of 1.5 volts. For example, effective voltage for a laptop computer can be as high as 24 volts. To obtain such voltages using fuel cell technology, individual fuel cells are connected in series, typically forming a fuel cell stack.
Current fuel cell stack designs utilize a bipolar plate to decrease the size, and increase the efficiency of said assembly. Instead of two current collectors, only one plate is used with a flow field cut into each side of the plate. That is, one side of the plate is used in the anode chamber of one fuel cell, while the other side is used in the cathode chamber of an adjacent fuel cell. The single plate may also serve to assist in the distribution of fuel on one side of the plate and an oxidant preferably from ambient air on the other side of the plate.
Bipolar plates are typically made of a gas-impermeable material, to prevent intermixing among the fuel on the anode side and the oxidant on the cathode side. Introduction of oxygen into the anode chamber of a fuel cell typically diminishes the performance of the cell, and may cause the methanol to oxidize completely, without contributing to the generation of electricity within the fuel cell system.
The bipolar plate is electronically conductive such that the electrons produced at the anode on one side of the bipolar plate can be conducted through the plate where they enter the cathode on the other side of the bipolar plate. Two end-plates, one at each end of the complete stack of cells, are connected via the external circuit.
One of the problems associated with fuel cell stacks using bipolar plates is that of eliminating gaseous effluent from the anode chamber. Prior art DMFC systems address this problem via a recirculation configuration system. In such a system, a gas separator incorporated in an effluent return line is used to remove gases from anode effluent fluids. The gas separator separates carbon dioxide from the unused fuel solution and exhausts carbon dioxide.
Although prior art recirculation configurations address some of the problems of handling anode effluent (conserving unused methanol fuel and rendering the fuel supply impervious to rapid changes in power demands of the fuel cell) these systems typically incorporate discrete auxiliary equipment to do so, including but not limited to gas separators and other components that separate liquids from gases. This auxiliary equipment consumes volume and adds to the overall materials and assembly costs, rendering re-circulating DMFC systems less feasible for portable power and electronics applications. Moreover, in fuel cell stack systems, gas separators must be used to ensure the performance of the stack and the system as a whole. Thus, the cost of the fuel cell stack increases dramatically in view of such additional requirements.
Therefore, it would be desirable to provide an apparatus and method for removing anode effluent gas from a fuel cell of a fuel cell stack where liquids may be separated from gases within the stack without adding additional volume or components.